1. Field of the Invention
The present invention relates to an antilock mechanism, and specifically to an antilock mechanism of a type in an automobile brake system which detects the locking condition of the wheels using a wheel velocity detector and a vehicle velocity detector, and which when the occurrence or signs of excessive slipping due to excessive brake action are detected, operates an electromagnetic valve and reduces the the brake pressure, thus suppressing and controlling the braking force to an optimum level, regardless of brake pedal depression.
2. Description of the Prior Art
Various types of antilock mechanisms have been previously proposed. For example, in an antilock mechanism as described in Japanese Patent Laid-Open Publication No. 49-28307, a normally open inlet valve is provided in the main channel between the hydraulic pressure source (master cylinder) operated by the action of the brake pedal and the wheel cylinder in the brake device, and a normally closed drain valve is provided in the drain channel between the wheel cylinder and the brake fluid tank. Each of the inlet and drain valves are electrically operated based on a detection signal from an antilock detection means.
In such a device, during non-antilock operation, power is not supplied to either the inlet valve or the drain valve so that working fluid flows into the wheel cylinder according to the action of the brake pedal.
On the other hand, during antilock operation, that is when the fluid pressure of the brake fluid is to be reduced due to the excessive braking, power is supplied to the inlet valve and drain valve. The inlet valve closes and the drain valve opens so that working fluid at the wheel cylinder side returns to the brake fluid tank and is then pumped back to the master cylinder.
When the pressure of the brake fluid is to be raised again, electrical power is cut so that the inlet valve opens and the drain valve closes. Moreover, to hold the brake fluid pressure constant, power is supplied only to the inlet valve to close the valve, and the drain valve remains closed to keep the brake pressure constant. Thus, an antilock mechanism of this type can be controlled in three modes, pressurize, pressure release, and constant pressure, but it requires two valves operated with an electrical supply, specifically inlet and drain valves. As a result, this requires more parts, more installation steps and assembly times, and thus poses the problem of higher cost.
An antilock mechanism which resolves the above problems and simplifies device construction with a single electromagnetic valve has been proposed and disclosed, for example, in U.S. Pat. No. 4,715,666. A device disclosed in this patent is shown in FIGS. 1 and 2. In place of the electromagnetically operated inlet valve in main channel 3 connecting hydraulic pressure source 1 and wheel cylinder 2, a flow control valve 5 of non-electromagnetically operating, three-port, two-position type is provided, which is operated by return spring 4 and hydraulic pressure. Also provided is a normally closed, two-port, two-position electromagnetic drain valve 7 in drain channel 6. According to this device, the combination of flow control valve 5 and electromagnetic drain valve 7 provides a two-mode antilock control of pressurizing and pressure releasing. As shown in the figure, flow control valve 5 is arranged such that spool 12 is inserted inside housing having a bore 11 so as to slide freely in the axial direction and is biased by return spring 4. The inlet port 13, first outlet port 14, and second outlet port 15 are each provided perpendicular to the axis. The inlet port 13 is provided for flowing therethrough working fluid from the master cylinder in the bore 11. The first outlet port 14 is provided so as to flow working fluid out to the wheel cylinder, and second outlet port 15 is provided so as to flow working fluid through drain valve 7 to the working fluid storage tank.
Also, drain valve 7 has a movable valve member located close to the second outlet port 15, i.e., located in the upstream side of the sealing portion defined by the valve member and a valve seat.
In the flow control valve according to U.S. Pat. No. 4,715,666 as described before, when air bubbles are captured in return spring chamber 17, such bubbles are easily crushed by the pressure. Thus, when the pressure rises rapidly at inlet port 13, spool 12 may undesirably move into the return spring chamber 17 toward the antilock operating position as the air bubbles are crushed. Thus, the rapid pressurization may become impossible. Furthermore, when the antilock operating position as shown in FIG. 2 is established, spool 12 is shifted to the left, when viewed in FIG. 2, so that the return spring chamber 17 is substantially isolated from the drain channel 6 except for a narrow variable orifice formed at an edge 20, and thus, the bubbles stay still in the chamber 17 during the antilock operation. Also, during the non-antilock operation, the spool 12 is shifted to the right, as shown in FIG. 1, so that the working fluid actively flows through the main channel 3 to wheel cylinder 2 as indicated by arrows, and no active flow is produced through the return spring chamber 17. Thus, the bubbles are still captured in the return spring chamber 17. Thus, according to the prior art antilock mechanism, it is very difficult to make the bubbles escape from the return spring chamber 17, once they are caught in the chamber 17.
Furthermore, the difficulty of bubble escaping with this construction is not improved even when the orientation of the flow control valve is inverted, or when the drain valve is activated by electromagnetic action. Thus, when air remains in the return spring chamber 17, the movement of the spool 12 becomes erratic and so the performance of the flow control valve.
Moreover, the housing of the flow control valve requires an opening on at least one end. Thus, it is necessary to block this open end with two plugs, one for solenoid and another for flow controller which are in orthogonal relationship to each other, requiring more parts and greater assembly time, and thus incurring increased costs.
Moreover, in the re-pressurizing process with the electromagnetic drain valve closed, it is necessary to keep the leakage of fluid between the spool and the bore as little as possible, so that the amount of fluid flow to the wheel brakes will not become greater than the amount of flow determined by the return spring and orifice. To this end, it is necessary to greatly increase the manufacturing precision of the spool and the bore, resulting in the higher costs.